In radio communication systems in recent years, modulation modes that employ two mutually orthogonal baseband signals have come into use as a means of raising the efficiency of utilization of frequency. These two baseband signals are referred to as an I signal and a Q signal. Because the I signal and Q signal are modulated at the time of transmission, an orthogonal mixer circuit is required in the reception circuit for demodulating such that the I signal and Q signal are mutually orthogonal.
FIG. 1 is a circuit diagram showing an orthogonal mixer circuit of the related art of the present invention (for example, refer to Patent Document 1). FIG. 2 is a circuit diagram that gives an abstract representation of the orthogonal mixer circuit shown in FIG. 1. Constructions that correspond to each other in FIG. 1 and FIG. 2 are given the same reference numbers.
In addition, FIG. 3 is a timing chart showing the waveforms of LO signals that are the oscillation signals of a local oscillator (LO) belonging to a reception circuit and the waveforms of a four-phase clock signal that is generated from the LO signals.
The LO signals have a duty ratio of 50% and are made up from four signals having phases that each differ by 90 degrees (LO_I signal, LO_Q signal, LO_IB signal, and LO_QB signal). In addition, the four-phase clock signal has a duty ratio of 25% and is made up from four signals having phases that each differ by 90 degrees (CLKI signal, CLKQ signal, CLKIB signal, and CLKQB signal).
In FIGS. 1 and 2, the orthogonal mixer circuit includes voltage-current conversion unit 201, RF path selection unit 202, load capacitors C201-204, and load resistors R201-R204. Load capacitors C201 and C202 and load resistors R201 and R202 make up a first load block, and load capacitors C203 and C204 and load resistors R203 and R204 make up a second load block.
An RF signal received by the reception circuit is applied as input to voltage-current conversion unit 201. The RF signal is a differential voltage signal of the radio frequency (RF) band.
Voltage-current conversion unit 201 converts the RF signal that was received to a differential current signal and supplies the result to RF path selection unit 202.
RF path selection unit 202 supplies the RF signal that was supplied from voltage-current conversion unit 201 to the first load block or the second load block according to the state of the four-phase clock signal. The RF signal is thus multiplied by the four-phase clock signal and an I signal and Q signal that are differential current signals of the intermediate frequency (IF) band. The I signal and Q signal are converted to differential voltage signals in load resistors R201-R204 and supplied as output.
Because the output destination of the RF signal is thus altered according to the state of the four-phase clock signal, the simultaneous flow of the I signal and Q signal through voltage-current conversion unit 201 is eliminated. Accordingly, the voltage-current conversion unit for use by the I signal and the voltage-current conversion unit for use by the Q signal can be shared.
When a voltage-current conversion unit for use by the I signal and a voltage-current conversion unit for use by the Q signal are separately provided, the problem arises that error occurs in the amplitudes of the I signal and the Q signal when there is variation in conversion gain between the voltage-current conversion units. The orthogonal mixer circuit shown in FIG. 1 and FIG. 2 is able to provide a solution to the above-described problem because the voltage-current conversion unit for use by the I signal and the voltage-current conversion unit for use by the Q signal can be shared.
In addition, despite divergences of the 90-degree phase difference between the LO_I signal and LO_Q signal, the phases of the four-phase clock signal that are generated by using these signals are characterized by having accurate 90-degree phase differences, as shown in FIG. 4. As a result, the use of a four-phase clock signal enables the generation of I signals and Q signals that are accurately orthogonal.